Presentation on theme: "Genetics and Inheritance Alleles and Appearance. Diploid zygote Haploid gametes."— Presentation transcript:
Genetics and Inheritance Alleles and Appearance
Diploid zygote Haploid gametes
Alleles and Appearance Remember: A gene is part of a chromosome. Each inherited characteristic is controlled by 2 copies of its gene. The 2 copies of the gene that determine a particular characteristic may be the same or different. Different forms of the same gene are called alleles.
Alleles and Appearance Remember that each gene occupies a specific site on a chromosome. Consider the gene for height in pea plants. Allele for tallAllele for dwarf What will the appearance of the pea plant be?
Alleles and Appearance Note Alleles are described as dominant or recessive. A dominant allele always shows up in an organisms appearance (masks the recessive allele). A recessive allele only shows up if there are two of them.
Alleles and Appearance Alleles are symbolised using letters Dominant alleles are given capital letters, while recessive alleles are given the same letter but in lower case. For example, in pea plants the dominant allele for tallness has the symbol T and the recessive allele for dwarfism has the symbol t.
Alleles and Appearance Dwarf = Recessive (t) Tall = Dominant (T) T T tt Tall Dwarf
Alleles and Appearance Notes Collect and complete the cut-out Symbols for Alleles, by choosing suitable symbols (letters) for the alleles.
Alleles and Appearance The two descriptions of an organisms characteristics that you need to know are: Genotype and Phenotype
Alleles and Appearance Notes The genotype of an organism is the alleles it carries for a particular characteristic. Genotypes are usually written as letter symbols, e.g. Tt.
Alleles and Appearance Notes The phenotype of an organism is the outward appearance of one of its characteristics and is usually described in words, e.g. tall. Phenotype is dependent upon genotype.
Alleles and Appearance Notes More vocabulary! A homozygous organism has two identical alleles of a gene, e.g. TT A heterozygous organism has two different alleles of a gene, e.g. Tt
Alleles and Appearance Notes Collect and complete the cut-out Genotypes and Phenotypes in Mice then Collect and complete the cut-out Predicting Phenotypes. In fruit flies, the grey body allele (G) is dominant to the black body allele (g).
Alleles and Appearance Think: Can different genotypes give the same phenotype? Can the same genotype give different phenotypes?
Genetics and Inheritance The monohybrid cross
The Monohybrid Cross Notes A monohybrid cross is a breeding experiment which follows the inheritance of just one characteristic, for example coat colour in mice.
The Monohybrid Cross Notes Organisms which are true breeding pass on the same characteristics to their offspring over many generations. This means that the offspring of true breeding black mice are always black and the offspring of true breeding white mice are always white.
The Monohybrid Cross Think! Will the genotype of a true breeding organism be homozygous or heterozygous?
The Monohybrid Cross Lets look at the example of mice. In mice, black coat (B) is dominant to white coat (b).
The Monohybrid Cross X P (parents) X F 1 (first filial generation) F 2 (second filial generation) Filial = son / daughter
The Monohybrid Cross X P X F1F1 F2F2 BB Gametes all B
The Monohybrid Cross What would the genotype of true breeding white mice be? What would be the genotype and phenotype of the F 1 and F 2 generation?
The Monohybrid Cross All offspring (F 1 ) bb Parents x bb x Second generation (F 2 ) bb Gametes all bb bb
The Monohybrid Cross X P X F1F1 F2F2
Another type of cross is when each parent is a true breeding individual, but each has a different phenotype. What would happen if you crossed a true breeding black mouse with a true breeding white mouse?
The Monohybrid Cross X P F2F2 X F1F1 bbBB Bb BBbb Bb BB bb bbBB
The Monohybrid Cross This can be shown in a punnet square: Bb BBBBb b bb Gametes
The Monohybrid Cross Notes Practice crosses: 1) Pea plants can be tall (T) or dwarf (t). If a true breeding tall plant was crossed with a true breeding tall plant what would the phenotype and genotyope of the F 1 and F 2 generations be?
The Monohybrid Cross Notes 2) Pea plants can be tall (T) or dwarf (t). If a true breeding tall plant was crossed with a true breeding dwarf plant what would the phenotype and genotyope of the F 1 and F 2 generations be?
The Monohybrid Cross Notes 3) R = red flowers r = yellow flowers A plant homozygous for red flowers is crossed with a plant that has yellow flowers. Work through the cross to the F 2 generation. What is the phenotype and genotype of the F 1 ? What is the phenotype and genotype ratios of the F 2 ?
The Monohybrid Cross Notes 4) R = red flowers r = yellow flowers A plant heterozygous for red flowers is crossed with a plant that has yellow flowers. Work through the cross to the F 1 generation. What is the phenotype and genotype ratios of the F 1 ?
The Monohybrid Cross Notes There is often a difference between the observed and predicted numbers of different types of offspring. This is because fertilisation is a random process involving the element of chance.
The Monohybrid Cross How can we tell if an organism is true breeding? If a mouse has a black coat, its genotype could be BB – true breeding, or Bb – not true breeding.
The Monohybrid Cross Notes A test cross is used to identify the genotype of an individual showing a dominant characteristic by crossing it with a homozygous recessive individual.
The Monohybrid Cross BB or Bb?
The Monohybrid Cross B B bb Bb All offspring: black B b bb Bb bb Bb bb Ratio = Black : White 1 : 1 X BB bb X Bb bb
The Monohybrid Cross Notes Now read Tp Human Inheritance and complete the Activity on p
Genetics and Inheritance Co-dominance
Co-dominance Notes Co-dominance is a pattern of inheritance where both alleles are expressed equally. Offspring from a cross between two true- breeding parents have a phenotype in between each parent.
Co-dominance – Think! In horses B = black coat W = white coat What would be the phenotype of a horse with the following genotypes: BB? WW? BW?
Co-dominance and Blood Group Blood group is determined by three alleles: A, B and O A and B are co-dominant to one another Both are completely dominant to O There are four blood group phenotypes: A, B, AB and O.
Co-dominance Notes A man with blood group A whose father was blood group O marries a woman with blood group AB. Draw a punnet square to show all the possible genotypes that their children could have. Which phenotype could not occur among their offspring?
Genetics and Inheritance Polygenic Inheritance
Variations between individuals fall into two categories: discontinuous or continuous Discontinuous characteristics fall into distinct categories – examples? Continuous characteristics show a range of differences from one extreme to the other – examples?
Discontinuous: Controlled by alleles of a single gene Single-gene inheritance Tongue rolling Roller Non-roller Blood Groups O B A AB
Continuous: Controlled by alleles of two or more genes Polygenic inheritance Height Skin Colour
Discontinuous data Continuous data No. of pupils Tongue rolling ability RollerNon-roller Distinct data can be presented as a bar graph. Pea seed shoot lengths (mm) No. of seeds Ranges of data can be presented as a histogram or line graph.
Polygenic Inheritance Notes A characteristic that is controlled by more than one gene and is expressed as a range of phenotypes is said to show polygenic inheritance. Include an example!
Genetics and Inheritance Organisms in genetic experiments
Qualities of an organism which make it suitable for genetic experiments are: Short life cycle, grow quickly Easy to breed, produce many offspring Easy to observe contrasting characteristics
Drosophila Drosophila (fruit flies) are suitable for carrying out genetic experiments as they are very small are easy to breed produce large numbers of offspring have a short life cycle have many different phenotypes, e.g. body colour are easy to distinguish, e.g. males have rounded abdomens, females have pointed.
Using Drosophila Notes The steps involved in setting up a cross to study the colour of the abdomen in Drosophila flies StepsNotes 1. Parents are chosen, one with a grey body and the other with an ebony body. Parents must be true breeding to ensure dominant alleles are not hiding recessive alleles.
Using Drosophila Notes StepsNotes 2. Breed the parents.This produces the F 1 generation.
Using Drosophila Notes StepsNotes 3. The F 1 phenotype is observed. This identifies the dominant allele.
Using Drosophila Notes StepsNotes 4. The F 1 generation is self-crossed. This produces the F 2 generation.
Using Drosophila Notes StepsNotes 5. The F 2 generation is examined and the number of each phenotype counted. This allows the phenotype ratio to be calculated.
Using Drosophila Notes StepsNotes 6. The experiment is repeated. This improves the reliability of the results.